The present invention relates to the field of gas turbines. It concerns a combustion chamber for a gas turbine, in which combustion chamber the hot combustion gases of a combustion zone are surrounded by inner walls which are cooled by cooling air, which is introduced through cooling air ducts outside the inner walls, which cooling air ducts are formed by an outer wall of the combustion chamber and the inner walls.
Such a combustion chamber is known, in the form of a secondary combustion chamber from, for example, the publication EP-A1 0 669 500 by the applicant.
In the combustion chambers, in particular the secondary combustion chambers, of conventional gas turbines, pressure vibrations or acoustic vibrations can occur in operation under certain conditions, these vibrations being located in the frequency range of several kHz, for example in the range from 2 to 6 kHz. Such vibrations are found to interfere with the operation and are therefore undesirable. One possibility for damping or suppressing such vibrations consists in providing fluid mechanics means in the combustion chamber which influence the flow of the hot gases in such a way that the acoustic vibrations are not excited or are only excited to a small extent. Another possibility consists in attaching, to the combustion chamber, so-called Helmholtz resonators which are coupled as elements damping the vibrations or making them disappear completely.
Various examples for the employment of Helmholtz resonators are known from the prior art. An annular combustion chamber for a gas turbine is described in the publication U.S. Pat. No 5,373,695. In this, individual Helmholtz resonators flushed with cooling air are arranged on the end surface near the burners. These Helmholtz resonators each comprise an external damping volume which is connected to the combustion chamber via a damping tube and is subjected to cooling air from the outside via a thin supply tube in order to prevent frequency detuning due to heat.
A gas turbine combustion chamber is described in the publication U.S. Pat. No. 5,644,918 in which, within the double shell supplying cooling air and surrounding the combustion chamber and at the end surface of the combustion chamber in the region of the burners, Helmholtz resonators 48 and 56 are formed by inserting additional partitions which are connected to the combustion chamber via contractions 50 and 58 but are otherwise completely closed so that there is no flow of cooling air through the resonator spaces.
Another solution which relates specially to a secondary combustion chamber is presented in the publication U.S. Pat. No. 5,431,018. A Helmholtz resonator flushed with cooling air surrounds, concentrically in this case, the fuel line which enters radially into the combustion chamber and through which the fuel for reheat is sprayed into the combustion chamber.
The known solutions operating with Helmholtz resonators are complex in design, can only be retrofitted to existing gas turbines with difficulty, occupy a substantial amount of space when a plurality of them are employed and are not compatible with cooling concepts in which the inner wall of the combustion chamber is cooled by cooling air introduced from outside. In addition, solutions with the use of Helmholtz resonators usually exhibit the disadvantage that their noise absorption profile covers a rather narrow band of the frequency range and cannot approximately cover the typically relevant range, mentioned above, of 2 to 6 kHz. Although the resonators can be differently tuned individually or in groups, which then leads to an inhomogeneous distribution of the absorption profile, such a solution has the inherent disadvantage that less power can be absorbed at a particular frequency.
Accordingly, one object of the invention is to provide a novel acoustically damped combustion chamber for gas turbines by means of a combination of Helmholtz resonators and a noise-absorbing perforated plate, which combustion chamber avoids the disadvantages of the known solutions and is distinguished, in particular, by little additional complexity and space requirement for the integrated resonators and, at the same time, permits effective cooling of the inner walls of the combustion chamber, and which exhibits the widest possible noise absorption profile in the frequency range.
The object is achieved in a combustion chamber of the type mentioned at the beginning wherein, at least in a partial region of the inner walls, the inner wall is formed from at least two perforated plates arranged essentially parallel to one another, wherein a first perforated plate borders directly on the cooling air ducts and is provided with a plurality of first openings through which cooling air from the cooling air ducts flows into a first intermediate volume located behind the first perforated plate, wherein a further perforated plate is arranged behind the first perforated plate, in the direction of the combustion zone, which further perforated plate is provided with a plurality of further openings, wherein the distance between the first perforated plate and the further perforated plate and the geometrical dimensions of the further openings are selected in such a way that the openings, together with intermediate volumes present between the perforated plates, form a plurality of mutually connected Helmholtz resonators and act as noise dampers for acoustic vibrations occurring in the combustion chamber, and wherein in addition further means are present which act to absorb noise. The core of the invention therefore consists in the fact that the combination of Helmholtz resonators with further noise-absorbing means leads to a wide noise absorption characteristic with little space requirement.
A first preferred embodiment of the invention is one in which, at least in a partial region of the inner walls, the inner wall is formed from three perforated plates arranged essentially parallel to one another, wherein a first perforated plate borders directly on the cooling air ducts and is provided with a plurality of first openings through which cooling air from the cooling air ducts flows into a first intermediate volume located behind the first perforated plate, which first intermediate volume is bounded, on the side facing toward the cooling air ducts, by the first perforated plate and, on the opposite side, by a second perforated plate, which second perforated plate is provided with a plurality of second openings, wherein a third perforated plate is arranged on the side of the second perforated plate facing away from the first intermediate volume, which third perforated plate is provided with a plurality of third openings and borders on the combustion zone, and wherein at least one of the perforated plates in addition acts to absorb noise. In other words, the core of the embodiment includes that one of the three perforated plates exhibits noise transmission which is as free from reflection as possible due to corresponding perforation arrangement or corresponding contraction ratio and that the combination and the geometric configuration of two further perforated plates creates a plurality of mutually connected Helmholtz resonators which cause a phase rotation. In addition, the complete absorption system is flushed by cooling air so that the resonators are stabilized thermally and in terms of frequency. The additional outlay to create the absorption system thenxe2x80x94if the large openings in the inner wall are already present in the case of existing effusion coolingxe2x80x94only includes the provision of two further perforated plates.
A second preferred embodiment of the combustion chamber according to the invention is one in which the contraction ratio, defined as the ratio between the area of the opening and the area located in front of it in the direction of the combustion zone, is essentially the same for the second or the third openings as the maximum Mach number occurring in the combustion space, which is defined as the ratio of the source velocity and the sonic velocity, and wherein the perforated plate provided with such openings acts in a noise-absorbing manner. In this way, either the second or the third perforated plate becomes the noise-absorbing plate because the transmission is adjusted to be echo-free, independently of frequency, by the selection of the perforations.
A third preferred embodiment of the combustion chamber according to the invention is one in which the distance between the first perforated plate and the second perforated plate and the geometrical dimensions of the second openings are selected in such a way that the second openings, in combination with the first intermediate space arranged between the first and the second perforated plates, provide Helmholtz resonators whose resonant frequency is essentially located within the range of the acoustic vibrations occurring in the combustion space, and wherein the third perforated plate is preferably also configured to absorb noise. Thus the third perforated plate leads to an echo-free transmission of the noise and the Helmholtz resonators located behind, in the noise propagation direction, displace its phase.
For the usual frequency values of the combustion chamber vibrations, in the range from 2 to 6 kHz, the second perforated plate has a thickness in the range from 0.1 to 1 cm, in particular preferably 0.6 cm, the area ratio of the acoustically relevant partial areas of the first intermediate volume and the areas of the second openings is in the range from 5 to 10, in particular preferably 8, the distance of the first from the second perforated plate is 0.1 to 1 cm, in particular preferably 0.6 cm, the product of the contraction ratio of the third openings and the maximum Mach number is in the range from 1 to 0.5, and the area ratio of the acoustically relevant partial areas in the combustion space and the acoustically relevant partial areas of the first intermediate volume is in a range from 1 to 2.
Another preferred embodiment of the combustion chamber according to the invention is one in which the distance between the first perforated plate and the third perforated plate and the geometrical dimensions of the third openings are selected in such a way that the third openings, in combination with the intermediate space arranged between the first and the third perforated plate, provide Helmholtz resonators whose resonant frequency is essentially located within the range of the acoustic vibrations occurring in the combustion space, and in which the second perforated plate is also preferably configured to absorb noise. This produces a noise-absorbing arrangement in which the noise-absorbing perforated plate is arranged in the actual damping volume of the Helmholtz resonators; this is found to be space-saving and nevertheless efficient.
The noise-absorbing arrangement can be provided with a particularly space-saving configuration and optimum cooling technology without any essential impairment from the acoustic point of view if, in accordance with a further embodiment, the second and the third perforated plates are joined directly together with no distance apart and have a uniform and concentric arrangement of the perforations in the two perforated plates or if the two perforated plates are even replaced by a single perforated plate with perforations of different diameters drilled from the two sides.